The present invention relates to an improved process intended for desulfurization of a gaseous feed containing hydrogen sulfide in order to eliminate more readily the elementary sulfur contained in a catalytic desulfurization solution and formed by chemical oxidation reduction reaction after a contacting stage. At least part of the catalytic solution from the contacting stage, containing elemental sulfur, is therefore recycled to the absorption stage under selected conditions allowing to increase the average size of the sulfur grains and notably to reduce the number of sulfur grains of very small size.
The process according to the invention is notably applied in a  less than  less than Redox greater than  greater than  type process for desulfurizing a gas containing at least hydrogen sulfide, where the gaseous feed is contacted with a catalytic solution. The catalytic solution comprises for example a polyvalent metal chelated by at least one chelating agent under suitable conditions for oxidation of the hydrogen sulfide to elemental sulfur and simultaneous reduction of the chelated polyvalent metal from a higher oxidation level to a lower oxidation level. After the absorption stage, a gaseous effluent practically free of hydrogen sulfide is recovered on the one hand and the catalytic solution at least partly reduced and containing elemental sulfur as a suspended solid is recovered on the other hand. At least part of the catalytic solution is recycled for example to the absorption stage so as to increase the size of the sulfur particles. The non-recycled part of the catalytic solution containing the sulfur is freed from the most part of the sulfur present as a suspension during a separation stage, then it is sent to a regeneration stage, air regeneration for example, prior to being used in the absorption stage.
The prior art describes many redox processes and associated devices allowing to eliminate hydrogen sulfide and to recover the elemental sulfur formed during the process.
The desulfurization process comprises for example the following two oxidation reduction stages:
during a first stage (absorption stage, oxidation reduction reaction), the hydrogen sulfide present in the gas to be processed reacts with chelated ferric ions according to the reaction:
H2S+2 Fe3+(chel)xe2x86x92S+2H++2 Fe2+(chel)xe2x80x83xe2x80x83(1)
The sulfur resulting from this chemical reaction has the form of grains of very small size. This sulfur is referred to as  less than  less than native sulfur greater than  greater than  in the description hereafter.
during a second stage (regeneration stage), the ferrous ions are reoxidized by the oxygen of air according to the reaction:
2 Fe2+(chel)+2 H++xc2xdO2xe2x86x922 Fe3+(chel)+H2Oxe2x80x83xe2x80x83(2)
These stages are notably described in patents U.S. Pat. No. 5,753,189, U.S. Pat. No. 4,859,436 and U.S. Pat. No. 4,532,118, and in patent applications WO-92/17,401, DE-34 44,252, but none of these documents describes or suggests recycling part of the catalytic solution at least partly reduced and containing elemental sulfur to the absorber in order to decrease the quantity of (small-size) native sulfur in the catalytic solution to be purified and regenerated. In fact, patent U.S. Pat. No. 5,753,189 describes recycling of part of the reduced catalytic solution freed from sulfur in order to control the sulfur concentration in the absorption zone.
The aqueous catalytic solutions used are generally very poor sulfur solvents. The presence of native sulfur grains of very small size can therefore disturb the course of these processes.
The devices generally comprise sulfur separation equipments using mechanical processes such as filtration, decantation or centrifligation. Sulfur grains of very small size can disturb this separation, for example by generating frequent filter clogging.
Furthermore, when the gas to be desulfurized is under high pressure, the process generally comprises a stage of preliminary expansion of the catalytic solution containing the sulfur prior to air regeneration. This expansion leads to degassing of the solution, which causes foaming problems that can be amplified by the presence of sulfur grains of very small size.
On the other hand, when the stage of regeneration of the catalytic solution at least partly reduced is carried out by contacting with air at atmospheric pressure, and the sulfur is most often recovered by flotation and/or decantation, the presence of sulfur grains of very small size can reduce the efficiency of this recovery.
The object of the invention is a new approach consisting in reducing the number of sulfur grains of very small size at least after the absorption stage in order to overcome notably the aforementioned drawbacks of the prior art. In order to obtain this result, at least part of the catalytic solution at least partly reduced and containing elemental sulfur is used and recycled to the absorption stage.
The invention relates to an improved process intended for desulfurization of a gaseous feed containing hydrogen sulfide, comprising at least the following stages:
a) contacting the gaseous feed with a catalytic solution comprising at least one polyvalent metal chelated by at least one chelating agent, under suitable conditions for oxidation of the hydrogen sulfide to elemental sulfur and concomitant reduction of the polyvalent metal from a higher oxidation level to a lower oxidation level (absorption stage),
b) recovering on the one hand a gaseous effluent substantially freed from hydrogen sulfide and, on the other hand, said catalytic solution at least reduced and containing elemental sulfur.
The process is characterized in that at least a fraction F1 of said catalytic solution at least reduced and containing solid elementary sulfur is recycled to absorption stage a) in order to reduce the number of sulfur grains of very small size.
Fraction F1 proper cannot correspond to the whole of the reduced catalytic solution coming from the contacting zone. This fraction F1 contains sulfur whose grain size can be selected.
According to an embodiment, the grain size of the sulfur grains suspended in the catalytic solution at least reduced after stage a) is for example determined and the quantity of the recycled fraction F1 of said solution is adjusted so as to maintain a particle grain size distribution within a given range.
The recycled quantity of the reduced catalytic solution containing the suspended sulfur can also be adjusted according to the quantity of sulfur in the feed. Generally, the larger the quantity of sulfur in the feed, the lower the recycle ratio. Advantageously, this recycled quantity containing the sulfur of selected grain size can represent between 1 and 95% by weight of the reduced solution, for example between 20 and 90%, preferably between 30 and 85% and more preferably between 50 and 80% by weight.
According to a feature of the process, the solid elemental sulfur is separated from the non-recycled fraction F2 of the catalytic solution (stage c)).
According to another feature of the process:
c) said reduced catalytic solution depleted in sulfur and preferably substantially freed from all of the sulfur can be at least partly regenerated,
d) at least part of the regenerated catalytic solution is recycled to a stage of contacting the regenerated solution with a gaseous feed consisting at least of hydrogen sulfide.
According to another feature, part of the reduced catalytic solution substantially freed from all of the sulfur can be recycled to contacting stage a).
According to an embodiment, the sulfur produced during separation stage c) can be washed with water in order to recover the catalytic solution imbibing it and the catalytic solution is processed by reverse osmosis in order to control the water content of said catalytic solution.
The sulfur produced during separation stage c) can be washed with water in order to recover the catalytic solution imbibing it and the catalytic solution can be processed by nofiltration in order to control the proportion of water and of ions of low molecular weight in the catalytic solution.
According to an embodiment, separation stage c) can be carried out by filtration and the grain size distribution range of the sulfur grains is selected according to the filtering means.
At least part of the reduced catalytic solution freed from the most part of the elemental sulfur and obtained after stage c) is expanded for example.
According to an embodiment, said reduced catalytic solution depleted in elemental sulfur, obtained after stage c), is fractionated into a fraction F4 and a fraction F3, and the non-expanded fraction F4 is recycled to absorption stage a).
The potential of said partly reduced catalytic solution is for example measured before the fractionation stage and fractions F3 and F4 are determined, the quantity of said fractions is controlled so as to maintain the ratio of the ferric ions to the ferrous ions between 0.1 and 100, preferably between 0.5 and 20.
Separation stage c) is for example carried out at a pressure ranging between 0.1 and 20 MPa.
The invention also relates to a device intended for desulfurization of a gaseous feed containing at least hydrogen sulfide, said device comprising at least one enclosure for contacting said gaseous feed with a catalytic solution containing at least one polyvalent metal chelated by at least one chelating agent, under suitable conditions for oxidation of the hydrogen sulfide to elemental sulfur and concomitant reduction of the chelated polyvalent metal from a higher oxidation level to a lower oxidation level, means intended for discharge of the gas freed from the most part of the sulfur and means intended for discharge of a mixture comprising catalytic solution F0 at least partly reduced and containing elemental sulfur grains.
The device is characterized in that it comprises at least one device for fractionating said solution F0 into at least two fractions F1 and F2, means for recycling said fraction F1 to the inlet of the contacting enclosure, means for controlling the grain size of the sulfur grains and means for controlling and/or regulating the quantity of fraction F1 recycled.
It can comprise a device for separating the solid elemental sulfur from said reduced catalytic solution, a zone intended for regeneration of said reduced catalytic solution, said regeneration zone comprising means intended for delivery of a gas comprising oxygen.
It can also comprise at least one expansion means located after said separation device and a flash drum.
The process and the device according to the invention are applied to desulfurization of a natural gas.
The process according to the invention notably affords the following advantages:
it favours collection of the  less than  less than native  greater than  greater than  sulfur formed during the absorption stage on the grains present in the recycled solution by aggregation or agglomeration, which reduces the number of small-size particles,
it favours operation of the sulfur filter plant located downstream from the absorber, the particles having a larger size. The sulfur  less than  less than cakes greater than  greater than  formed on the filter being thicker, the frequency of the declogging operations is reduced or the installed filtering surface is reduced,
it improves the filtering efficiency. The number of particles that can pass through the filtering surface is greatly reduced, notably at the beginning of the filtering operation when a precoat forms on the filtering surface,
it therefore improves the catalytic solution regeneration efficiency.
The process according to the invention is for example applied for desulfurizating a natural gas, refinery gases such as hydrogen-rich gases circulating in hydrotreatment plants, coke-oven gases,etc.